The present invention relates to radio frequency ("RF") attenuator circuits and more particularly to a PIN diode attenuator having improved linearity and return loss.
In various RF communication applications, such as cable television networks, amplifiers are used along a signal path to maintain signal strength. Various amplifiers are known, particularly for cable television ("CATV") applications, that sense ambient temperature and change the gain and slope of the amplifier to compensate for cable losses. Such amplifiers typically comprise bridged-T RF PIN diode attenuators and cable equalizers.
Various ways are known for providing the drive currents necessary to operate PIN diode attenuators and cable equalizers. For example, the PIN diodes can be driven by pairs of transistors, pairs of operational amplifiers ("op-amps"), or fixed resistors. Such known methods of driving PIN diodes suffer several disadvantages. One disadvantage is that they do not provide a linear change of attenuation with control voltage. Thus, the attenuation of the PIN diode network does not vary linearly with drive current, so that the resultant attenuation and/or equalization is nonlinear with respect to changes in the control voltage. As a result, attenuation and equalization is typically very sensitive at the ends of the control voltage range and relatively insensitive at the center of the range. This nonlinearity makes it difficult to design accurate thermally controlled attenuators and other PIN diode signal processing circuits. Prior attempts to deal with the nonlinearity has made the resulting circuits quite complex.
Another disadvantage of PIN diode attenuators is that they do not exhibit good return loss at all settings of attenuation. It is desirable to hold RF circuits at a constant impedance. In cable television applications, for example, a 75 ohm impedance throughout the signal distribution path is optimal. Variations from the optimal line impedance cause the RF signals to be reflected off of the receiving device. With lower return loss, greater reflection will occur, creating ghosts in a received television video signal. It is desirable to hold the return loss in a cable television system to about 18 dB (or greater) across the whole band.
The curve of theoretically correct resistance, for the PIN diode, in a bridged-T RF attenuator is nonlinear. The nonlinearity required of the attenuator element does not match the nonlinearity of a PIN diode RF resistance. As a result, prior art bridge-T RF PIN diode attenuators do not exhibit good return loss at all settings of attenuation.
It would be advantageous to provide an improved signal processing circuit, such as a PIN diode RF attenuator, slope control circuit, or equalizer, that exhibits improved linearity. It would be further advantageous to provide such a signal processing circuit that exhibits good return loss over the operating range of the device. The present invention provides an improved circuit with the aforementioned advantages.